For a long time, there’s been an interesting discrepancy between the capacity listed on a hard drive’s label and the capacity reported by the computer. For example, attaching a 250GB hard drive would show up in the system as having 232.74GB available. Many would chalk it up to “formatting.” While the formatting information takes up some space, 17GB is a little excessive for formatting data. So where did this other space go?

The real culprit here is the discrepancy between base-10 mathematics (how most of us count) and binary (aka “base-2″) counting. To drive manufacturers, a kilobyte was 1000 bytes, a megabyte was 1000 kilobytes and a gigabyte was 1000 megabytes.

However, computers don’t natively use base-10; they use a base-2 system. To them, a kilobyte is defined as 1024 (which is 2¹⁰) bytes, a megabyte is 1024 kilobytes, and a gigabyte is 1024 megabytes.

This methodology worked fine for many years; after all, 1024 isn’t TOO far off from 1000. As drive capacities increased, however, this became more and more pronounced. Drive manufacturers were defining “gigabyte” as 1,000,000,000 bytes (1000 x 1000 x 1000), while computers recognized a gigabyte as 1,073,741,824 bytes (1024 x 1024 x 1024). Every gigabyte added to a drive exacerbated the problem, adding 73,741,824 bytes to the discrepancy.

Snow Leopard, though, changes this. Instead of simply reporting the base-2 number for a unit of drive space, it converts it to an easier-to-understand base-10 number – the same way it is measured by drive manufacturers. In easier terms: a 500GB drive shows up as 500GB in the Finder, rather than 463.13GB.

Of course, this doesn’t mean that you magically get more drive space. You still have the same number of bytes (the base unit) to deal with. The number of bytes that make up larger increments has just changed. Of course, this change in measurement is applied across the board in the finder. All your files will seem “larger,” even though they all have the same number of bytes in them. For example, here’s a pair of screen shots of a folder in my music library.

These shots are of the same files, in the same folder, on the same drive. In 10.6, though, they’re reported as being “larger.” But are they? The main folder shows up as having 308,937,619 bytes in both systems. The only difference is the 10.5 uses base-2 for its measurement, and 10.6 uses base-10. In 10.5, a megabyte is 1,048,576 bytes. In 10.6, it’s an even 1,000,000. Divide 308,937,619 by both of those, and you can see how the Finder in each OS arrived at its figure.

This may be a bit confusing for a while – after all, we’ve kind of gotten used to things the way they were. There is a bright point, though: now you don’t have to ask where all that space went when you install or attach your new hard drive.

For more information, you can check out this Apple KnowledgeBase article.